1. Field of the Invention
Exemplary aspects of the present invention relate to a fixing device, an image forming apparatus, and a fixing method, and more particularly, to a fixing device for fixing a toner image on a recording medium, an image forming apparatus incorporating the fixing device, and a fixing method performed by the fixing device.
2. Description of the Related Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of a photoconductor; an optical writer emits a light beam onto the charged surface of the photoconductor to form an electrostatic latent image on the photoconductor according to the image data; a development device supplies toner to the electrostatic latent image formed on the photoconductor to render the electrostatic latent image visible as a toner image; the toner image is directly transferred from the photoconductor onto a recording medium or is indirectly transferred from the photoconductor onto a recording medium via an intermediate transfer belt; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.
Such fixing device is requested to shorten a warm-up time to warm up the fixing device and a first print time taken to output the recording medium bearing the toner image onto the outside of the image forming apparatus after the image forming apparatus receives a print job.
To address these requests, the fixing device may employ a thin fixing belt having a decreased thermal capacity and therefore heated quickly by a heater. As a first example, a pressing roller is pressed against a substantially tubular, metal heat conductor disposed inside a loop formed by the fixing belt to form a fixing nip between the pressing roller and the fixing belt. The metal heat conductor is supported by a support that divides the interior of the loop formed by the fixing belt into two compartments. The heater situated inside one of the two compartments heats the fixing belt via the metal heat conductor. As the fixing belt and the pressing roller rotate and convey a recording medium bearing a toner image through the fixing nip, the fixing belt and the pressing roller apply heat and pressure to the recording medium, thus fixing the toner image on the recording medium. Since the heater heats the fixing belt via the metal heat conductor that faces the entire inner circumferential surface of the fixing belt, the fixing belt is heated to a predetermined fixing temperature quickly, thus meeting the above-described requests of shortening the warm-up time and the first print time.
As a second example, the pressing roller is pressed against a nip formation pad disposed inside the loop formed by the fixing belt via the fixing belt to form the fixing nip between the pressing roller and the fixing belt. A reflector situated inside the loop formed by the fixing belt divides the interior of the loop formed by the fixing belt into two compartments: a first compartment accommodating the nip formation pad and a second compartment accommodating a heater. Thus, the heater is disposed opposite the nip formation pad via the reflector. Since the fixing belt is heated by both light radiated from the heater toward the fixing belt directly and light radiated from the heater and reflected by the reflector, the fixing belt is heated to a predetermined fixing temperature quickly, thus meeting the above-described requests of shortening the warm-up time and the first print time.
However, the support of the first example and the reflector of the second example that divide the interior of the loop formed by the fixing belt into the two compartments may obstruct uniform heating of the fixing belt in a circumferential direction thereof. For example, an opposed portion of the fixing belt disposed opposite the compartment accommodating the heater is heated by the heater directly. Conversely, a non-opposed portion of the fixing belt not accommodating the heater is not heated by the heater directly. Accordingly, before the fixing belt rotates for a substantial time, the temperature of the fixing belt may vary in the circumferential direction thereof between the opposed portion and the non-opposed portion. Such variation in the temperature of the fixing belt may lead to variation in thermal expansion of the fixing belt in the circumferential direction thereof, which may result in deformation, such as bending, warp, and buckling, of the fixing belt. Accordingly, the deformed fixing belt may produce the deformed fixing nip where the fixing belt and the pressing roller may not apply heat and pressure to the recording medium bearing the toner image uniformly, resulting in faulty fixing of the toner image on the recording medium.